Band pass tuner



Aug. 20, 1935. TT 2,012,030

BAND PASS TUNER Filed March 3, 1933 W4 v51 ENGTH RAY/STANCE INVENTOR- ALFRED? WITTS ATTORN EY- no matter what station the receiver is tuned Patented Aug. 20, 1935 t 1 039]. fi r t "BA P EUN i A lfred Thomas Witts Surrey,- England,': assigfir to Radio Corporation of America, a corporation of Delaware 7 'Application'Mahh3,1933; Serial No, 659,48g .111

' ln Gieat'Britain Marcn. -1v,.1 e32 1: I 1 -1 A Claiin sf j (Cline-44 V v M pli fic'ation of a radio receiveryan aeriaI A, which This invention relatesto frequency selection means for use in high frequencyelectrical cir cults; and although not limited to its application theretofis particularly suitablefor'use in con- 5 nection with ceivers;

"As is well'knownit'is very desirable in abroadcast radio receiver to ,be able to tune said receiver to receive, and substantially, uniformly to amplify, all the frequencies infastriotly' limited band of frequencies of width corresponding to the widthpi the modulationside bands of a broadcast transmitting station' and to be able to move this band up and down within the tunable range of the receiver. In other words iti's very desirable that "the tuning of a broadcast -radio rea radio receiver shall have ;What is generally termed band pass tunin eshall be'such that to, it will receive the full mo dulation' width of v the said station (say abcut 1O,'O0O cycles) but shall reject all frequencies" outside this band; It is therefore; common to provide radio receiv ers with sic-called tunableband.passfilters; and, although these are fairly'asatisfactory' in prac tice, l :noWn forms oi b'and pass; filters present'the serious disadvantage that" their resistance is not constant over the wide range iof wave lengths normally required for broadcast reception. This disadvantage is well known "and various proposals have been made to obviate i't, bu t all the proposals so farm'adeh'ave offered more or less serious disadvantages and have not beenfen tirely successful in practice, it being generally the case'that at some part of theirequency spectrum the choice hasjto'b made between reduced amplification ratiofor increased width; of the acceptedirequency bond.

' The pres'ent'i fveiiti Q hasffor itsob'ject price vid a tunable bandpass filter'arrang'ement which is an'explanatory graph. Referring to the said Fig. 1, which represents the first stage of am- .i'esistanc; is constant.

te-w and the new v'alvemay not be. of precisely.

shall accept substantially thesa ne widthof bar d' AG. is coupledto'an inductanoe L1 in aband pass filter-comprising two tuned circuits LiCLILrQg .which arecoupled to one another by condensers C304. The variable condensers'C1C2-are ganged together, i. e; arranged ior simultaneous adjust merit,- the 'fi1ter-as'so*fardescribed" being of a kindwell known per se. The output of the filter is Tappliedbetw'een the control g rid and cathode of an amplifier valveV' (shown as '01 I the "screen grid-type)- and; negative resistance "is-finjected into the filter bya circuit includinga-cofl L's-and a variable condenser C5; coilQLrbeing coupled to the. coilLaihjthe band; "filter proper.

The "amount-, of negative resistance injected {is 'controlledby the condenser C5 whicm 'asisshown,

the" condenser "beingso constructed". i. e..hav-

in'gsuch" a' law of capacity setting that at all positions *of tuning' given bythe condensers'pr and C2 thel'total resistancei. e. the resistance presented b'y the filter proper plus the negative If desired the c'oilll g may be coupled to the coils Li and L2 instead or. as shown,- to the-coil L2 only. As will be appreciated it'may be necessary to change thevalve'V from time to time,

e.'j g; when it wears cutftoreplace it bya new the'same constants and} characteristics ofthe valveit replaces.- "Such changingfoi the valveV might result therefore inupsetting thefganging v of the tuning arrangement and-in f rder -to r vide ior an adjustment to take care ofth'is pos 'sibility the 'condenser; C5 is preferably "prcvided with a socalled'Vtfimmingg condenser,which may 'be adjusted' f or the purpose imparting the necessary compensation.

C5 is; suchthat the Control of the negative resistrance injected into'theband pass" filter. from its minimum value must be, substantially of the same order oflniagnitudfe "as the increase in positive re sistance'of the loand jpa ss filter; Consequently the actual shapev will" varyaccording to the positiveresistance characteristic of the band filter,

but the standard types of variable condenser plates may be. used for this purpose.

- In the example shown in the drawing the .re-

sistance frequency curve is given in Fig. 2. Inorder to get a negative resistance variation similar to curve I) an ordinary straightline frequency condenser sufiices; the condenser may have the usualadjustable split end plates. Such a condenser is in common use for effecting satisfactory tracking of ganged circuit tuning condensers. As the maximum value of negative, resistance is required at the highest frequency to which the filter is tuned, the setting of C5 must be a maximum at that point. Therefore, the ganging of C5 with the tuning condensers C1 and C2 must give maximum value, of C5 when the setting of C1 and C2 is a minimum.

For other types of band pass filters with posi tive resistance frequency curves different from,

that shown in Fig. 2, the various standard ,types of condenser plates can be used; for example,

with square law and logarithmic'capacity variation with angle of rotation. The desired modification to bring the negative resistance curve into substantial agreement with-the positive re- 7 sistance curve of each particular type of filter. can be effected by adjustment of the split end plate. s V

As regards the low end of capacity setting; this need not be of any specified value so long as the control exercised on the negative resistance enables ,any increase in positive resistance of the filter to be compensated. The amountof negative resistance injected must be zero at the low est point of the resistancefrequency curvecor-l responding to the particular band pass filter emp y d.

Referring to Fig. 2 of the drawing the curve a represents the-resistance of a normal so-called double-capacity-coupled filter,. i. e. a filter as shown in Fig. l but without the negative resist+ ance provision. As will be seen the resistance falls with increasing wave length. The require merit tobe met is that of maintaining the over,-

all resistance-constant at the value which it has at the-highest wave lengthin the tuning range, i. e. at the'value yf. To put the mattenin'another way, the straight line 0 is the resistance characteristic which is required. This character istic c is obtained by so designing the circuit for injecting negative resistance that the amount of 7 negative resistance injected is as given by the curve b, the curve b being such that the ordinate" at any pointalong the abscissa line 0X is equal and opposite to the ordinate of the curve a minus the ordinate of the curve 0 at that point. The invention is, of course, of general. application to all forms of band pass filters and. vari-' ous methods of injecting negative resistance may 7 be resorted to. For example, a dynatron may be coupled to the filter in place of the reactive winding L3 or Fig.1, the potential of the third electrode of the dynatron being operated about a suitable point onthe operating curve by means of;a potentiometer or equivalent arrangement which is uni-controlled with the tuning. means for the filter proper. Alternatively the potentials applied to the dynatron maybe constant and the coupling between said dynatron. and the filter proper variedby means uni-controlled withfthe filter tuningmeans. i

In yet a further modificationiluse is'madef j the negative resistancecharacteristic of a specially provided screen grid valve. In this 'arrangement means uni-controlled with the filter tuning means are provided for altering the potential of the anode or the potential or the control grid, or the potentials of the screen grid and anode together and in vthisway the required amount of negative resistance is injected into the band pass filter proper at each position of tuning.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

I 1.- A tuning circuit arrangement suitable for use in radio receivers and for like purposes, and

"comprisingin combination a tunable band pass filter arrangement and means, controlled simultaneouslywith the normally provided means for varying:the tuning of said filter, for injecting negative resistance into said filter, the means for injecting negative resistance being such that the amount of negative resistance injected substantiallycompensates for variations in resistance of Y cuit, a band-pass filter including a plurality of coupled gang-controlled tuned circuits for coupling. said aerial circuit to the input circuit of an amplifiervalve, an inductance in series with an adjustable condenser connected in circuit between the plate and cathode of said amplifier valve, said .inductance being coupled to an inductance in the band-pass filter, and meansfor uni-controlling. said adjustable condenser with tuning condensersprovided for varyingthe tuning of the band-pass filter to maintain a constant resistance characteristic, over the receiver tuning range.,,

4. In combination with a radio freq'uency band passnetwork of the type including a pair of tuned circuits, both resonant to .a common'signalirequency, and both circuits being coupled by at least one ,untuned reactance to provide a band pass characteristic, eachof said tuned circuits inc1ud-' inga variable tuning condenser, means for unicontrol1ing'both condensers to tune said band pass network over a desired signal range, a signal source coupled to one of said tuned circuits, an electron discharge'tube having its input electrodes coupled to theother tuned circuit, a. regenerative feedback path'- coupling the anode circuit of said tubeto the, input circuit thereof and including anelement coupling said path to the saidband pass network, saidpathincluding a variable condenser mechanically coupled with said uni-controlled tuning means whereby sufiicient negative resistance isinjected into said network as said tuning: means is varied to maintain the resistanceof said network substantially constant over said tuningrange.

ALFRED THOMAS Wrrrs. 

